Do roof tiles fail after a period of time?

I was looking up at the roof the other day and saw that there
appeared to a couple of tiles missing. I then looked out the dormer
window in the loft and saw that two half tiles were sitting in the
rain gutter.

House is a 1928 build and to date have not experienced any roof
problems.No one has been near the roof in the last 8 years so physical
abuse can be ruled out.

Given the age of the roof tiles, can I expect others to fail in the
near future and might I be faced with retiling the whole roof? Or
should I get someone round to just replace the pair. I am fortunate in
that I have a dozen left over from when the dormer was put in 8 years
ago.

VT